1. Field of the Invention
This invention relates to an in-focus position detecting apparatus incorporated in an optical instrument such as a still camera or a video camera, and in particular to a so-called active-type in-focus position detecting apparatus in which light is projected from the apparatus side toward an object to be photographed and the then reflected light from the object to be photographed is photoelectrically detected to thereby detect the in-focus state with respect to the object to be photographed.
2. Description of the Prior Art
A TTL active-type in-focus position detecting apparatus has already been proposed by Japanese Laid-open Patent Application No. 155832/1979.
FIG. 1 of the accompanying drawings shows the optical arrangement and principle of such a TTL active-type in-focus position detecting apparatus. In FIG. 1, reference numeral 1 designates a focusing lens, reference numeral 2 denotes the optic axis of the focusing lens, reference numeral 3 designates the predetermined imaging plane of the focusing lens, reference numeral 4 denotes a light emitting element such as a semiconductor laser (LD) or an infrared light emitting diode (IRED), and reference numeral 7 designates a light receiving element such as a charge coupled device (CCD) or a silicon photodiode (SPC). The light emitting element 4 and the light receiving element 7 are disposed at positions equivalent to the center of the optic axis of the predetermined imaging plane 3. A light flux 5 emitted from the light emitting element 4, reflected by a mirror 4' and transmitted through the off-axis light projecting aperture of the focusing lens 1 is projected onto the surface 8 of an object to be photographed. The light flux 6 reflected by the surface of the object to be photographed is transmitted through the off-axis light receiving aperture of the focusing lens 1, is reflected by a mirror 7' and enters the light receiving element 7.
In FIG. 1, (a) shows the near focus state, (b) shows the in-focus state, and (c) shows the far focus state. In the in-focus state of (b), the spot of the emitted light flux 5 is sharply imaged at the center of the optic axis on the surface 8 of the object to be photographed, and the spot of the reflected light flux 6 also is sharply imaged at the center of the optic axis on the light receiving element 7. In contrast, in the states of (a) and (c), the spots of the light fluxes deviate from each other while blurring in opposite directions on the surface 8 of the object to be photographed and on the surface of the light receiving element 7. Thus, if the direction of displacement of the center of gravity of the signal light flux on the surface of the light receiving element is discriminated, detection of the direction and in-focus of the photo-taking lens can be accomplished.
In such a construction, the accuracy of distance measurement depends on the interval between the aperture of the emitted light flux 5 and the aperture of the reflected light flux 6, and the limit of the measurable distance depends on the areas of said two apertures. That is, to enhance the automatic focus detecting performance, the interval between and the areas of the two apertures need be made great, and this provides the factor for making the diameter of the focusing lens 1 great. That is, this system has a merit that it requires no interlocking mechanism between the photo-taking lens system and the in-focus detecting system and no parallax occurs between the photo-taking lens system and the in-focus detecting system, while the in-focus detecting system is contained in the photo-taking lens and the automatic focus detecting light flux is projected and received through the focusing lens portion and this unavoidably leads to the bulkiness of the entire system, particularly the bulkiness of the focusing lens portion which results from the necessity of causing a light flux larger than the photo-taking light flux to be transmitted. Further, in the aforementioned Japanese Laid-open Patent Application No. 155832/1979, there is a possibility that ghost which may cause malfunctioning in detecting the in-focus state may occur. That is, it is conceivable that the emitted light flux is scattered by the lens surface of the focusing lens portion and the scattered light may enter the light receiving element to cause malfunctioning.